Cells for the electrolysis of fused salts



Jan. 1, 1963 A. KOEHL ETAL 3,071,532

CELLS FOR THE ELECTROLYSIS OF FUSED SALTS Filed March 8, 1960 a a E 2" NINVENTORS Goflfr/Zed HJeI/emann Ernst Kaegi by kw AT TORNFYS 3,dll,532

Patented Jan. 1, 1%63 3,ll71,532 CELM; FUR THE ELE CTRQLYSES til FUSEDSALTS Anton Koehl, Gattfried Fuellemann, and Ernst Kaegi, Montltey,Switzerland, assignors to Ciba Limited, Basel, witzerland, a Swiss firmFiled Mar. 3, H60, Ser. No. 13,586 Claims priority, applicationSwitzerland Dec. 7, 1959 3 Claims. (Cl. 204-443) This invention relatesto cells for the electrolysis of fused electrolytes, and particularly tocells used in the manufacture of sodium by electrolysis of common salt.

In large scale manufacturing processes, electrolysis of molten metalsalts, and particularly of alkali halides, has been carried out for manyyears. The manufacture of metallic sodium from common salt or mixturesthereof with other salts, such as calcium chloride, barium chloride andthe like, has become of especial importance and has resulted in thedevelopment of many types of cell. The majority of these cells consistessentially of a cylindrical steel container, lined with a ceramicmaterial, through the bottom of which enters a central graphite anodesurrounded by an annular cathode. The space between the anode and thecathode is divided by an annular diaphragm which supports suitableapparatus for collection of the deposited materials, such as, forexample, chlorine and sodium. A cell of this type is described in US.Patent No. 1,501,756 to James Cloyd Downs (patented luly 15, 1924).

Although such cells generally have been fairly successful, they stillhave certain disadvantages, since the operation of a fused saltelectrolytic cell involves technical problems, due to high workingtemperatures, corrosiveness of salts employed in the processes and thenature of the final products, the solution of which presentsdifiiculties.

One of these problems arises in the ceramic lining of such electrolyticcells. It is difiicult indeed to find any ceramic material which, forprolonged periods, can resist attack due to the effects of molten saltsused in the process and due also to the products of electrolysis. Apartfrom the fact that the ceramic materials used are expensive, thefabrication of the lining itself tends to be a complicated and costlyoperation. Further, during renewal of the lining, these cells cannot beoperated making economical utilisation of the plant difiicult.

The use of a diaphragm also gives rise to difiiculties in that materialsused in manufacturing these diaphragms do not stand up to the demandsmade of them during operation of cells. The metal screens used asdiaphragms have to be replaced at relatively frequent intervals duringwhich replacement operation of the cells must cease, the latter must bepartly dismantled and may even have to be emptied, thereby furthercomplicating the operation of the plant.

However, no cell is at present known which can be operated without adiaphragm and which makes as efficient use of supply energy and currentas does a cell using a diaphragm.

Conventional cells for the electrolysis of fused salts thus tend to haveconstructional complications and to be relatively expensive to maintainand operate; it is an object of this invention to provide an improvedcell.

According to the present invention, a cell for the electrolysis of fusedsalts includes a container within which a cathode structure surrounds ananode structure, the anode structure comprising a supporting column anda plurality of anode plates removably and electrically connectedthereto, the plates being spaced apart from the column and each having aface directed toward the cathode structure.

The anode plates may be mutually spaced apart and may be arranged toform an annular or polygonal ring around the column. Conveniently, thecolumn may have radially extending arms to which the plates areattached. Preferably, the anode structure is so dimensioned andconstructed, and together with the cathode structure so arranged withinthe container, that the electrolyte can freely circulate within thecontainer. Further it is advantageous that the effective opposingsurfaces of the cathode and anode are not separated by any intermediatesurface, such as, for example, a diaphragm.

By way of example, a cell embodying the invention, and suitable for theproduction of sodium by the electrolysis of common salt, will bedescribed in greater detail and with reference to the accompanyingdrawings, in which:

FIGURE 1 is a sectional side view of a cell embodying the invention, and

FIGURE 2 is a section on the line A-A of FIG- URE 1.

FIGURE 1 shows a cell comprising a container in which is mounted ananode structure including a centrally disposed column 1 2, made ofgraphite, and a number of plates 12. forming the working anode and whichare renewable and attached to the central column llll by separatesupport arms 14. The anode plates 12 and support arms 14 are also madeof graphite. Opposite and facing anode plates 12 there is an annularcathode 16, made of iron. The column 16 is mounted in the bottom 18 ofthe container and is also used to supply current to the anode plates 12,the positive side of a power supply being connected to a terminal 20located on column llll on the underside of the container.

As shown in FIGURE 1, the bottom of the container consists of a shell 22lined with a corrosion resistant material; this lining also forming thelower floor 24 of the cell. A drainpipe 25 is provided in the floor 24through which the cell can be emptied.

A side wall 28, made of iron and forming the inner wall of the cell, isinserted into a groove 26 in the bottom of the container. To providethermal insulation the side wall 28 is enclosed in a thermallyinsulating layer 30. The cathode 16 is supported by fourcircumferentially spaced bars 32 which pass, electrically insulated,through a collector channel 34. The bars 32 also act as current supplyleads for the cathode 16.

The channel 3 3, which is of annular form and open on its underside, islocated above the cathode l6 and is immersed in the electrolyte 46. inoperation of the cell, channel 34 collects sodium deposited at thecathode 16. As shown on the right hand side of FlGURE 1, this channelsupports at one point on its periphery a collector chamber 38 withinwhich sodium collects above the electrolyte, the latter having a higherdensity, so that the so- (hum can be discharged through passage 40. Atanother point on the channel 34 there is an inlet opening through whichthe cell can be replenished with electrolyte. The collector channel 34-is made, as is the wall of the cell, of iron.

A collector dome for chlorine released at the anode surfaces rests uponthe collector annulus 34. The lower edge 44 of the dome is immersed inthe electrolyte 46 thereby sealing oil": the gas space from the exteriorof the cell. At the highest point of the dome 42 there is an aperture 48through which the collected chlorine can be discharged; if desired otherapertures may be provided in the dome 42 through which electrolyte alsomay be admitted to the cell. The dome 42 preferably is made of acorrosion resistant ceramic material.

The anode plates are so attached to the column 10 that they may berenewed. For this purpose, as is shown in FIGURE 1, the support arms 14have pins 13 and 15, of which pins 13 fit into corresponding holes inthe column 19 and pins 15 into the anode plates 12. It is to beunderstood that other methods of attachment may also be used, forexample, the pins of arms 14 may be screw threaded and holes tapped inplates 12. The anode plates are arranged to form a circular or polygonalring, located within the annular cathode, in general with the anodeplates 12 being mutually spaced apart. The supporting arms 14 are ofadequate length to ensure that an area of sufiicient section isavailable at the rear side of the anode plates in order to allowcirculation of the electrolyte around the anode plates, as indicated byarrows 50, to take place. The cathode is likewise spaced from the wallsof the container. Further, the anode plates the cathode are displacedfrom the bottom of the Such a design al ows a free flow of a melt withinthe cell, as indicated by arrows t) and 52, which is well defined andwhich particularly in the space between the anode plates and the cathodeis free from turbulence. Chlorine forming at the anode plates and sodiumdeposited at the cathode can rise up without any turbulence causingmixing of these two substances. The thermal convection taking placebetween the anode and cathode aids a comparatively rapid flow of theelectrolyte in an upwards direction which is of importance in so far asit prevents overheating of the electrolyte in the vicinity of theelectrodes and thereby increases the eiiiicency of the cell. The risingmelt thus circulates between the anode column and the anode plates andalso between the cathode and the side walls of the cell Where it becomescooled. The favorable flow pattern thus achieved also makes it possibleto arrange the cathode and anode working surfaces directly opposite eachother without separation by a diaphragm or other intervening surface.The efiiicency of the cell, as regards consumption of supply current andenergy, however, is comparable with that obtained by use of aconventional diaphragm equipped cell.

Apart from the fact that the design or" anodes described above resultsin a favorable flow pattern, it also leads to a substantial saving inmaterial and labor in the operation of the cells. In actual fact onlythe anode plates are subject to substantial wear during operation of thecell, the supporting arms and anode column being subject to much lesswear. After a given period of operation it is suflicient, therefore, torenew only the worn anode plates, whereupon the cell can be refilled andoperated again. This substantially simplifies maintenance operations andreduces the time during which the cell need be out of operation duringsuch maintenance.

Elimination of the need for a diaphragm also results in a simplificationof construction and reduction of cost in the operation of the cell.

Another advantage is that the side walls are not lined and that liningis restricted to the floor of the cell. This is important as the ceramiclining of a fused salt electrolytic cell is a difficult operation. Noknown material will resist for a prolonged period the chemical attack bythe contents of the cell. The lining must therefore be renewed atcomparatively short intervals of time, thereby increasing the cost ofoperation of the cell. In the present case, normal maintenance work islimited to the replacement ofthe top layer of the floor lining which mayconsist of ceramic slabs or a compressed material.

The elimination of the ceramic lining is also advantageous from thepoint of view of operation of the cell. In fact, the attack by the meltand the sodium on the ceramic material forms decomposition products inlined cells which collect on the floor of the cell forming there aviscous mass which in time reduces the circulation of the melt, becomeelectrically conductive and hence causes short circuits which graduallyreduce the efiiciency of the cell, necessitating its drainage andcleaning. In the case of a cell constructed in accordance with thisinvention, the ceramic lining is limited to the bottom of the cell, andthus the surface exposed to attack by the melt is greatly reduced. Theformation of decomposition products is also reduced, resulting in a longoperating life at a substantially constant rate of yield of the cell.

Another advantage having a similarly favorable effect is the fact thatboth the working anodes and the cathode are arranged at some distanceabove the bottom of the cell, since some amount of deposit can form onthe floor without being in the actual neighbourhood of the electrodes.This features helps to diminish dangers of a short circuit.

The thermal insulation layer is provided on the outside of the cell thusnot coming in contact with the electrolyte. Hence any desired type ofinsulating material can be used since it is not subject to anyparticular chemical requirement. It is possible also to vary the heatinsulation of a cell during operation as required, thereby ensuringoptimum, or near optimum temperature conditions for any particular cell.

Another advantage resulting from the elimination of the inside lining ofthe cell is the very simple and efficacious construction of the cell.The sidewall is not attached rigidly to the bottom but rests in asealing groove so that it can be lifted up, together with the chlorinedome, cathode and sodium collector without difiiculty from the floor ofthe cell, without involving the dismantling of the thermal insulation.This provides ready access to the floor portion of the cell, so thatboth inspection and maintenance of the cell are greatly facilitated.Thus, for example, it is possible within a very short time to lift upthe upper portion of the cell, remove any sediment collected on thebottom, replace the upper portion, refill the cell and put it intooperation again.

What is claimed is:

1. A cell for the electrolysis of fused salts, comprising a containerhaving a bottom to hold electrolyte, a central support column in saidcontainer, a plurality of anode plates in said container having theiractive faces disposed vertically and being spaced apartfrom each otherand from said container bottom and forming together a ringshapedstructure concentrically arranged with respect to said central supportcolumn, said anode plates being removably attached to said centralcolumn by means of radial arms extending from said column in spoke-likeconfiguration, said radial arms located substantially equidistantlybetween the top and bottom of said container, a ring-shaped cathodeinside said container concentrically surrounding said anode plates toprovide a free space between said oathode and anode plates, said anodeplates being spaced with respect to each other and said cathode platewhereby fluid flow mutually therebetween is possible, said centralcolumn piercing said container bottom and providing electricalconnection means with said anode plates and a current supply.

2. A cell as described in claim 1, wherein said container comprises abase portion forming the bottom of the container, an upstandingcylindrical iron side wall, a thermal insulation provided only on theexterior surface of said side wall, and a covering dome for the gasesliberated at said anode plates, each of the three parts representing aself supporting unit easily detachable from the other container parts;said cell further comprising an an- .nular collecting channel forgathering the material deposited on the cathode, said channel locatedabove said cathode and extending downward below the electrolyte fluidlevel maintained during normal cell operation.

3. A cell for the electrolysis of fused salts comprising a container, aplurality of anode plates in said container having vertically-orientedactive faces, a support column for said anode plates, said columnpiercing the bottom of said container and providing electricalconnection between said anode plates and an electrical current source,saidanode plates being removably connected with said support column bymeans of radial arms extending from said column in spoke-likeconfiguration, said radial arms being of substantially equal length andlocated substantially equidistantly between the top and bottom of saidcontainer, said anode plates forming a concentric ring with respect tosaid support column, and a ring-shaped cathode concentrically andexteriorly arranged with respect to said column and said anode plates toprovide a free space between said anode plates and said cathode.

References Cited in the file of this patent UNITED STATES PATENTSCastner May 12, Blackrnan Sept. 22, Ward et al. Mar. 29, Hardy et a1.Mar. 19, McNitt Dec. 4,

1. A CELL FOR THE ELECTROLYSIS OF FUSED SALTS, COMPRISING A CONTAINERHAVING A BOTTOM TO HOLD ELECTROLYTE, A CENTRAL SUPPORT COLUMN IN SAIDCONTAINER, A PLURALITY OF ANODE PLATES IN SAID CONTAINER HAVING THEIRACTIVE FACES DISPOSED VERTICALLY AND BEING SPACED APART FROM EACH OTHERAND FROM SAID CONTAINER BOTTOM AND FORMING TOGETHER A RINGSHAPEDSTRUCTURE CONCENTRICALLY ARRANGED WITH RESPECT TO SAID CENTRAL SUPPORTCOLUMN, SAID ANODE PLATES BEING REMOVABLY ATTACHED TO SAID CENTRALCOLUMN BY MEANS OF RADIAL ARMS EXTENDING FROM SAID COLUMN IN SPOKE-LIKECONFIGURATION, SAID RADIAL ARMS LOCATED SUBSTANTIALLY EQUIDISTANTLYBETWEEN THE TOP AND BOTTOM OF SAID CONTAINER, A RING-SHAPED CATHODEINSIDE SAID CONTAINER CONCENTRICALLY SURROUNDING SAID ANODE PLATES TOPROVIDE A FREE SPACE BETWEEN SAID CATHODE AND ANODE PLATES, SAID ANODEPLATES BEING SPACED WITH RESPECT TO EACH OTHER AND SAID CATHODE PLATEWHEREBY FLUID FLOW MUTUALLY THEREBETWEEN IS POSSIBLE, SAID CENTRALCOLUMN PIERCING SAID CONTAINER BOTTOM AND PROVIDING ELECTRICALCONNECTION MEANS WITH SAID ANODE PLATES AND A CURRENT SUPPLY.